The history of evolution is periodically marked by explosions in biodiversity, as groups of species try out a wide range of shapes and sizes. With a new analysis of two such adaptive radiations in the fossil record, researchers have discovered that these diversifications proceeded head-first.

By analyzing the physical features of fossil fish that diversified around the time of two separate extinction events, scientists from the University of Chicago and the University of Oxford found that head features diversified before body shapes and types. The discovery disputes previous models of adaptive radiations and suggests that feeding-related evolutionary pressures are the initial drivers of diversification.

"It seems like resources, feeding and diet are the most important factors at the initial stage," said lead author Lauren Sallan, graduate student in the Department of Organismal Biology and Anatomy at the University of Chicago. "Strange heads show up first  crushing jaws, animals with big teeth, with long jaws  but they're all pretty much attached to the same body."

Adaptive radiations underlie the evolution of dominant and diverse groups. After a major disruption, such as an extinction event, surviving species diversify into a myriad variety of forms. Modern examples of this diversity are the fish family of cichlids, with more than 1,000 documented species, or "Darwin's finches" of the Galapagos Islands, which exhibit many different beak types.

Evolutionary biologists have used these living species to propose at least two models of how adaptive radiations work. One model proposes a single "burst" of divergence followed by a long period of relative stability. Another, sometimes known as the "general vertebrate model," introduced the idea of staged divergences, with habitat-driven changes in body type preceding diversification of head types.

However, these models had not yet been tested with the rich data sets available in the fossil record.

"There hadn't been any tests of these things using fossils," said Sallan, a graduate student in the laboratory of University of Chicago Professor Michael Coates. "You have all these analyses of diversification, yet not one of them goes back to the fossil record and says what's happening at this time period, and the next time period, and the one after that."

Sallan and co-author Matt Friedman, PhD, lecturer in paleobiology at the University of Oxford and a former member of Coates' laboratory, looked at two different adaptive radiations in the fossil record. The first was the explosion of ray-finned fishes after the Hangenberg extinction, an event 360 million years ago that decimated ocean life on Earth. The second group was the acanthomorphs, a group of fish that exhibited a burst in diversity around the time of the end-Cretaceous extinction that ended the age of dinosaurs.

In both datasets, the researchers used a method called geometric morphometrics to quantify differences in features such as body depth, fin position and jaw shape between species. Crucially, Sallan and Friedman separated head features from body features in their analysis, to better detect the timing of when each compartment showed a burst of diversity in the record.

The results of the two analyses were in agreement: Diversification in cranial features preceded diversification in body types. Unusual head features such as jaws lined with sharp teeth or blunt teeth for crushing appeared before diverse body shapes on a spectrum from slender and eel-like to broad and disc-shaped.

"We have these two entirely separate radiations, and in both of them the pattern is heads first. So feeding might be more important to diversification than habitat use," Sallan said. "It's against both the adaptive radiation model and the proposed stage model."

The pattern detected with the new analyses suggests that the appearance of new sources of food drives a burst of diversity before species begin to change to adapt to new habitats.

"Ecological limits are taken away," Sallan said. "There's more opportunity out there, more available resources, and they're taking advantage of that. Later, they're taking advantage of specializing to new habitats. So it's not something within the animals themselves; it's more opportunity that matters."

While the new study offers two distinct examples of head-first diversification separated by hundreds of millions of years, the universality of the model remains to be conclusively proven.

"Evolution is really complex, and it's not really clear that there should be only one model," Sallan said. "It might be that this model might apply to fishes in certain time periods, or might apply to vertebrates, but a lot more investigation is needed to see whether that is actually true."

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User comments : 8

There is no Missing link.The concept suggested by darwin would be displayed in the fossil record, a gradual History of sorts written in stone, but that is not what the Fossil record shows. There are thousands of Species existing today, and there is no historical list of changes back from each of these species. Its not there. Why? because the theory of Evolution is false.

There is no Missing link.The concept suggested by darwin would be displayed in the fossil record, a gradual History of sorts written in stone, but that is not what the Fossil record shows. There are thousands of Species existing today, and there is no historical list of changes back from each of these species. Its not there. Why? because the theory of Evolution is false.

Later, they're taking advantage of specializing to new habitats. So it's not something within the animals themselves; it's more opportunity that matters.

The author is contradicting himself here. Firstly, it's now become clearer that there IS some epi-genetic memory that survives into later generations and that it influences the growth of those. Secondly, one should note that it's already existing information from within the fish itself that gets modified into a slightly different form, resulting in a new appearance. But the new information is the result of existing codes being switched on or off. It's a case of "If X then build Y". One can debate this point till kingdom come. Future research will show who is closer to the truth here.Thirdly, note that the fish remain FISH. They don't change into lizards or birds or dinosaurs.

There is no Missing link.The concept suggested by darwin would be displayed in the fossil record, a gradual History of sorts written in stone, but that is not what the Fossil record shows. There are thousands of Species existing today, and there is no historical list of changes back from each of these species. Its not there. Why? because the theory of Evolution is false.

There have been many thousands of intermediate forms discovered. The creationist assertion that there is not is the equivalent of putting ones fingers in ones ears and whistling. We don't find fossils of every species, and don't expect to, because fossilisation is a rare occurrence - few creatures that die become fossils. The geological column presents a great opportunity for you creationists to falsify evolution. According to the flood myth, you should find dinosaurs buried with people, dogs, mammoths, rabbits etc. I'm not going to hold my breath though.

Row Boat Fair It should read/study pages 129 to 177 in Morris & Sherwin's Fossil Recod (2010). Conclusion? There are no "transitional forms" (but please, read the pages first!) Secondly, teeth have always been teeth, "The past few years have witnessed a remarkable flurry of research on the origin or origins of vertebrate teeth. While this work is progressing, the details of when, where, why, and how teeth first appeared still elude consensus." - Ungar (2010) p. 73. This includes fish teeth. Finally, as predicted by creationists, fish have always been fish - even as "far back" as the lower Cambrian! The "earliest" fossils of fish are not transitional. Also see Benton (2005) Fig. 7.13 BTW - another darwinian myth has collapsed - fishless Silurian seas. Not any more (Nature 458:413). I enjoy reading about these 'extinction events' - darwinists are getting closer & closer - screaming & kicking - to the Truth (Gen. chs 6-9).

Row Boat Fair It should read/study pages 129 to 177 in Morris & Sherwin's Fossil Recod (2010). Conclusion? There are no "transitional forms" (but please, read the pages first!) Secondly, teeth have always been teeth, "The past few years have witnessed a remarkable flurry of research on the origin or origins of vertebrate teeth. While this work is progressing, the details of when, where, why, and how teeth first appeared still elude consensus." - Ungar (2010) p. 73. This includes fish teeth. Finally, as predicted by creationists, fish have always been fish - even as "far back" as the lower Cambrian! The "earliest" fossils of fish are not transitional. Also see Benton (2005) Fig. 7.13 BTW - another darwinian myth has collapsed - fishless Silurian seas. Not any more (Nature 458:413). I enjoy reading about these 'extinction events' - darwinists are getting closer & closer - screaming & kicking - to the Truth (Gen. chs 6-9).

Actually creation and science are the same thing.The hypothesis , of 'evolution' is based on non human interferance. And as they say in the article this has not been tested. This same idea was proposed by Darwin with his Finches. The results were that the y were still Finches and when conditions reverted back to normal,so did the Finches.Also in creation, inteligent breeding is also possible, like we have breed dogs fro example. The other very important , method of creation is the idea of creating a life, and from that life taking DNA and body tissues,to create a new life. This is not just an idea I cooked up, the creator said he did this. It is the example of Adam and Eve. Eve was created from Adam not just a swab of DNA but tissues as well. In animals this would leave an records of previous populations. And would also explain lines of descent the 'evolutionary' scientists find. 'This is my 'Patterns of Creation'found herepatternsofcreation.weebly.com

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